This invention relates to improvements in external combustion engine systems of the type which generate power by the expansion of a non-burning gas. More particularly the invention relates to improvements in a uniflow-type piston-driven engine for increasing the thermal efficiency thereof.
The increasing demand for pollution-free automobile engines and other power plants, and the more recent awareness of gasoline shortages, have both indicated a strong need for replacement of the internal combustion engine. The steam engine, able to capitalize on the low emission advantages of external combustion and the simplified mechanics and drive train made possible by high starting torque and a reversible engine, is one very likely successor. Other possibilities include external combustion engines utilizing freon (CCl.sub.2 F.sub.2), thiophene [(CH).sub.4 S] or other similar elastic fluids. In any automobile engine, it would appear that pistons must be utilized rather than turbines, since turbines require very high volumes, lack low speed torque and work best at relatively constant high speeds, thereby requiring substantial gear reduction.
One disadvantage, at least until now, of the external combustion piston engine (or expansion engine), is that it has lacked sufficient thermal efficiency to make it a practical alternative for automotive use, although it is virtually emission-free and is capable of using kerosene or cheap grades of fuel oil. Probably the most significant development in recent years with respect to improving the efficiency of external combustion piston engines has been the development of the "uniflow" principle of exhaust valving, primarily applied to steam engines, whereby the exhaust is arranged so that the steam or other expansible fluid flows from the end of the cylinder to exhaust ports located near the center, and does not reverse its direction of flow during exhaust as is the case with older types of external combustion engines. This elimination of exhaust flow through inlet ports was important because it substantially eliminated a particular type of energy loss known to those skilled in the art as "initial condensation", thereby markedly improving the efficiency of the engine. Despite the improved efficiency of the uniflow-type engine, however, great energy losses still exist. One of the most significant of these is the energy loss resulting from incomplete expansion of the steam or other gas admitted into the cylinder caused by the release of the gas from the engine at the end of the stroke at too high a pressure. Not only does such incomplete expansion tend to decrease the amount of usable energy which can be extracted from the gas, but it also places a greater load on the condenser in a recycling system since the returning gas is at a relatively higher energy level and the size and power requirements of the condenser and its associated blower must also be proportionately greater. Other substantial causes of energy loss are friction and the inertia and leverage inefficiencies of the standard crankshaft drive system used in conventional engines. Friction losses are particularly aggravated by the crankshaft design because the connecting rod necessarily transmits driving force at considerable angles between the piston and the crank, thereby tending to cock the piston alternately in either of two directions against the cylinder wall as it reciprocates and causing increased friction. The lack of any substantially straight-line force transmission through the connecting rod and the resultant loss of torque also substantially hinders the efficiency of present external combustion piston engines.